With seven months left in the campaign, no candidate running for reelection -- save President Barack Obama -- has been targeted by outside interests more than Sen. Sherrod Brown (D-Ohio). Over $5.1 million has been spent on television ads opposing Brown in the 2012 cycle, according to data provided by a Senate Democratic campaign operative. The top spenders are the U.S. Chamber of Commerce, which spent $2.7 million, and 60 Plus Association, the conservative group that opposes health care reform, which spent another $1.4 million. Karl Rove's Crossroads GPS and Concerned Women for America Legislative Action Committee have also spent heavily in the race.

Despite this onslaught, however, Brown has yet to receive large-scale support from either progressive or labor groups, with whom he has been popular throughout his career.

McCain also said this week that he might expand his campaign efforts in Ohio beyond the Senate race, perhaps to aid GOP congressional candidate Samuel "Joe the Plumber" Wurzelbacher, who was a notable figure in McCain's failed presidential campaign.

"I am certainly glad when anybody wants to run to serve their country, and I'm sure if he asks for my help, I'm glad to do it," McCain told the Toledo Blade on Monday.